Various exposure apparatuses have been hitherto used, for example, when semiconductor elements or liquid crystal display elements are produced by means of the photolithography step. At present, a projection exposure apparatus is generally used, in which an image of a pattern formed on a photomask or reticle (hereinafter generally referred to as “reticle”) is transferred via a projection optical system onto a substrate (hereinafter referred to as “sensitive substrate”, if necessary) such as a wafer or a glass blade applied with a photosensitive material such as photoresist on its surface. In recent years, a reduction projection exposure apparatus (so-called stepper) based on the so-called step-and-repeat system is predominantly used as the projection exposure apparatus, in which a sensitive substrate is placed on a substrate stage which is movable two-dimensionally, and the sensitive substrate is moved in a stepwise manner (subjected to stepping) by using the substrate stage to repeat the operation for successively exposing respective shot areas on the sensitive substrate with the image of the pattern formed on the reticle.
Recently, a projection exposure apparatus based on the step-and-scan system (scanning type exposure apparatus as described, for example, in Japanese Laid-Open-Patent Publication No. 7-176468, corresponding to U.S. Pat. No. 5,646,413), which is obtained by applying modification to the stationary type exposure apparatus such as the stepper, is also used frequently. The projection exposure apparatus based on the step-and-scan system has, for example, the following merits. That is, (1) the projection optical system is easily produced because a large field can be exposed by using a smaller optical system as compared with the stepper, and a high throughput can be expected owing to the decrease in number of shots because a large field is exposed. Further, (2) an averaging effect is obtained owing to relative scanning for the reticle and the wafer with respect to the projection optical system, and thereby it is possible to expect improvement in distortion and depth of focus. Moreover, it is considered that the scanning type projection exposure apparatus will be predominantly used in place of the stepper, because a large field will become essential in accordance with the increase in the degree of integration of the semiconductor element, which is 16 M (mega) at present and will become 64 M for DRAM, 256 M, and 1 G (giga) in future as the progress proceeds along with times.
With this type of projection exposure apparatus, alignment between the reticle and the wafer needs to be performed highly precisely prior to exposure. To carry out this alignment, the wafer is provided with a position detecting mark (alignment mark) formed (or exposure transferred) by a previous photolithographic process. By detecting the position of this alignment mark, the exact position of the wafer (or a circuit pattern on the wafer) can be detected.
Alignment microscopes for detecting the alignment mark are roughly classified into the on-axis type for detecting the mark through a projection lens, and the off-axis type for detecting the mark without allowing the detecting light pass through a projection lens. With regard to a projection exposure apparatus with an excimer laser light source, which would be predominant in this field, an alignment microscope of the off-axis type is optimal. This is because the projection lens has been corrected for chromatic aberration due to exposure light, so that the on-axis type cannot condense alignment light, or if it could, an error due to chromatic aberration would be marked. An alignment microscope of the off-axis type, on the other hand, is provided separately from the projection lens; therefore, free optical design is possible without regard for such chromatic aberration, and various alignment systems can be used. For example, a phase contrast microscope or a differential interference microscope may also be used.
When the sensitive substrate is subjected to exposure by using the scanning type projection exposure apparatus, the so-called complete pre-measurement control method has been carried out as follows as described, for example, in Japanese Laid-Open Patent Publication No. 6-283403 corresponding to U.S. Pat. No. 5,448,332. That is, all detecting points included in one array provided on a front side in the scanning direction with respect to an exposure field are used as sample points. All values of focus positions at the sample points are previously measured before exposure, followed by the averaging process and the filtering process. The autofocus and the autoleveling mechanisms are controlled in an open manner during the exposure in consideration of phase delay. Concurrently with the foregoing operation, an inclination in the non-scanning direction is determined by means of the least square approximation method from the measured values of the focus positions at the respective sample points in the one array described above to perform the leveling control in the non-scanning direction in accordance with the open control.
Such a projection exposure apparatus is principally used as a mass-production machine for semiconductor elements or the like. Therefore, the projection exposure apparatus necessarily required to have a processing ability that how many sheets of wafers can be subjected to the exposure process for a certain period of time. That is, it is necessarily required for the projection exposure apparatus to improve the throughput.
In this context, in the case of the projection exposure apparatus based on the step-and-scan system described above, when a large field is exposed, the improvement in throughput is expected because the number of shots to be exposed on the wafer is decreased as described above. However, since the exposure is performed during movement at a constant velocity in accordance with synchronized scanning for the reticle and the wafer, it is necessary to provide acceleration and deceleration areas before and after the constant velocity movement area. As a result, if a shot having a size equivalent to a shot size of the stepper is exposed, there is a possibility that the throughput is rather decreased as compared with the stepper.
The outline of the flow of the process in such a projection exposure apparatus is as follows.    (1) At first, a wafer load step is performed, in which a wafer is loaded on a wafer table by using a wafer loader.    (2) Next, a search alignment step is performed, in which the position of the wafer is roughly detected by using a search alignment mechanism. Specifically, the search alignment step is performed, for example, on the basis of the contour of the wafer, or by detecting a search alignment mark on the wafer.    (3) Next, a fine alignment step is performed, in which the position of each of the shot areas on the wafer is accurately determined. In general, the EGA (enhanced global alignment) system is used for the fine alignment step. In this system, a plurality of sample shots included in the wafer are selected beforehand, and positions of alignment marks (wafer marks) affixed to the sample shots are successively measured. Statistical calculation based on, for example, the so-called least square method is performed on the basis of results of the measurement and designed values of the shot array to determine all shot array data on the wafer (see, for example, Japanese Laid-Open Patent Publication No. 61-44429, corresponding to U.S. Pat. No. 4,780,617). In this system, it is possible to determine the coordinate positions of the respective shot areas with high accuracy at a high throughput.    (4) Next, an exposure step is performed, in which the image of the pattern on the reticle is transferred onto the wafer via the projection optical system while successively positioning the respective shot areas on the wafer to be located at exposure positions on the basis of the coordinate positions of the respective shot areas having been determined in accordance with the EGA system or the like described above and the previously measured baseline amount.    (5) Next, a wafer unload step is performed, in which the wafer on the wafer table having been subjected to the exposure process is wafer-unloaded by using a wafer unloader. The wafer unload step is performed simultaneously with the wafer load step (1) described above in which the exposure process is performed. That is, a wafer exchange step is constructed by the steps (1) and (5).
As described above, in the conventional projection exposure apparatus, the roughly classified four operations are repeatedly performed by using one wafer stage, i.e., wafer exchange→search alignment→fine alignment→exposure→wafer exchange.
The throughput THOR [sheets/hour] of such a projection exposure apparatus can be represented by the following expression (1) assuming that the wafer exchange time is T1, the search alignment time is T2, the fine alignment time is T3, and the exposure time is T4.THOR=3600/(T1+T2+T3+T4)  (1)
The operations of T1 to T4 are executed repeatedly and successively (sequentially) as in T1→T2→T3→T4→T1 . . . . Accordingly, if the individual elements ranging from T1 to T4 involve high speeds, then the denominator is decreased, and the throughput THOR can be improved. However, as for T1 (wafer exchange time) and T2 (search alignment time), the effect of improvement is relatively small, because only one operation is performed for one sheet of wafer respectively. As for T3 (fine alignment time), the throughput can be improved if the sampling number of shots is decreased in the case of the use of the EGA system, or if the measurement time for a single shot is shortened. However, on the contrary, the alignment accuracy is deteriorated due to shortened T3. Therefore, it is impossible to easily shorten T3.
On the other hand, T4 (exposure time) includes the wafer exposure time and the stepping time for movement between the shots. For example, in the case of the scanning type projection exposure apparatus based on, for example, the step-and-scan system, it is necessary to increase the relative scanning velocity between the reticle and the wafer in an amount corresponding to the reduction of the wafer exposure time. However, it is not allowed to increase the scanning velocity without consideration because the synchronization accuracy is deteriorated.
With the apparatus using an off-axis alignment microscope, such as the projection exposure apparatus with the excimer laser light source which would be predominant in this field, it is not easy to improve the controllability of the stage. With this type of projection exposure apparatus, there is need to precisely control the position of the wafer stage, without Abbe's error, during exposure of the mask pattern through the projection optical system and during alignment, thereby to achieve highly precise superposition. For this purpose, it is necessary to set a constitution in which the measuring axis of the laser interferometer passes through the center of projection of the projection optical system and the center of detection of the alignment microscope. Furthermore, neither the measuring axis passing through the center of projection of the projection optical system nor the measuring axis passing through the center of detection of the alignment microscope should be interrupted in the moving range of the stage during exposure and in the moving range of the stage during alignment. To satisfy this requirement, the stage necessarily becomes large in size.
Important conditions for such a projection exposure apparatus other than those concerning the throughput described above include (1) the resolution, (2) the depth of focus (DOF: Depth of Focus), and (3) the line width control-accuracy. Assuming that the exposure wavelength is λ, and the numerical aperture of the projection lens is N.A. (Numerical Aperture), the resolution R is proportional to λ/N.A., and the depth of focus (DOF) is proportional to λ/(N.A.)2.
Therefore, in order to improve the resolution R (in order to decrease the value of R), it is necessary to decrease the exposure wavelength λ, or it is necessary to increase the numerical aperture N.A. Especially, in recent years, semiconductor elements or the like have developed to have high densities, and the device rule is not more than 0.2 μm L/S (line and space). For this reason, a KrF excimer laser is used as an illumination light source in order to perform exposure for the pattern. However, as described above, the degree of integration of the semiconductor element will be necessarily increased in future. Accordingly, it is demanded to develop an apparatus provided with a light source having a wavelength shorter than that of KrF. Representative candidates for the next generation apparatus provided with the light source having the shorter wavelength as described above include, for example, an apparatus having a light source of ArF excimer laser, and an electron beam exposure apparatus. However, the case of the ArF excimer laser involves numerous technical problems in that the light is scarcely transmitted through a place where oxygen exists, it is difficult to provide a high output, the service life of the laser is short, and the cost of the apparatus is expensive. The electron beam exposure apparatus is inconvenient in that the throughput is extremely low as compared with the light beam exposure apparatus. In reality, the development of the next generation machine, which is based on the principal viewpoint of the use of a short wavelength, does not proceed so well.
It is conceived to increase the numerical aperture N.A., as another method to increase the resolution R. However, if N.A. is increased, there is a demerit that DOF of the projection optical system is decreased. DOF can be roughly classified into UDOF (User Depth of Focus: a part to be used by user: for example, difference in level of pattern and resist thickness) and the overall focus difference of the apparatus itself. Up to now, UDOF has contributed to DOF in a greater degree. Therefore, the development of the exposure apparatus has been mainly directed to the policy to design those having a large DOF. Those practically used as the technique for increasing DOF include, for example, modified illumination.
By the way, in order to produce a device, it is necessary to form, on a wafer, a pattern obtained by combining, for example, L/S (line and space), isolated L (line), isolated S (space), and CH (contact hole). However, the exposure parameters for performing optimum exposure differ for every shape of the pattern such as L/S and isolated line described above. For this reason, a technique called ED-TREE (except for CH concerning a different reticle) has been hitherto used to determine, as a specification of the exposure apparatus, common exposure parameters (for example, coherence factor σ, N.A., exposure control accuracy, and reticle drawing accuracy) so that the resolution line width is within a predetermined allowable error with respect to a target value, and a predetermined DOF is obtained. However, it is considered that the following technical trend will appear in future.    (1) In accordance with the improvement in process technology (improvement in flatness on the wafer), the difference in pattern level will be progressively lowered, and the resist thickness will be progressively decreased. There will be a possibility that the UDOF may change from an order of 1 μm→0.4 μm.    (2) The exposure wavelength changes to be short, i.e., g-ray (436 nm)→i-ray (365 nm)→KrF (248 nm). However, investigation will be made for only a light source based on ArF (193) in future. Further technical hurdle is high. Thereafter, the progress will proceed to EB exposure.    (3) It is expected that the scanning exposure such as those based on the step-and-scan system will be predominantly used for the stepper, in place of the stationary exposure such as those based on the step-and-repeat system. The step-and-scan system makes it possible to perform exposure for a large field by using a projection optical system having a small diameter (especially in the scanning direction), in which it is easy to realize high N.A. corresponding thereto.
In the background of the technical trend as described above, the double exposure method is reevaluated as a method for improving the limiting resolution. Trial and investigation are made such that the double exposure method will be used for KrF exposure apparatus and ArF exposure apparatus in future to perform exposure up to those having 0.1 μm L/S. In general, the double exposure method is roughly classified into the following three methods.    (1) L/S's and isolated lines having different exposure parameters are formed on different reticles, and exposure is performed for each of them on an identical wafer under an optimum exposure condition.    (2) For example, when the phase shift method is introduced, L/S has a higher resolution at an identical DOF as compared with the isolated line. By utilizing this fact, all patterns are formed with L/S's by using the first reticle, and L/S's are curtailed for the second reticle to form the isolated lines.    (3) In general, when the isolated line is used, a high resolution can be obtained with a small N.A. as compared with L/S (however, DOF is decreased). Accordingly, all patterns are formed with isolated lines, and the isolated lines, which are formed by using the first and second reticles respectively, are combined to form L/S's. The double exposure method described above has two effects of improvement in resolution and improvement in DOF.
However, in the double exposure method, the exposure process must be performed several times by using a plurality of reticles. Therefore, inconveniences arise in that the exposure time (T4) is not less than two-fold as compared with the conventional apparatus, and the throughput is greatly deteriorated. For this reason, actually, the double exposure method has not been investigated so earnestly. The improvement in resolution and depth of focus (DOF) has been hitherto made by means of, for example, the use of an ultraviolet exposure wavelength, modified illumination, and phase shift reticle.
However, when the double exposure method described above is used for the KrF and ArF exposure apparatuses, it is possible to realize exposure with up to 0.1 μm L/S. Accordingly, it is doubtless that the double exposure method is a promising choice to develop the next generation machine aimed at mass-production of DRAM of 256 M and 1 G. Therefore, it has been expected to develop a new technique for improving the throughput which is a task of the double exposure method as a bottleneck for such a purpose.
In this context, if two or more operations of the four operations, i.e., the wafer exchange, the search alignment, the fine alignment, and the exposure operations can be concurrently processed in parallel, it may be possible to improve the throughput as compared with the case in which the four operations are sequentially performed. For this purpose, it is premised that a plurality of substrate stages-are provided. The provision of a plurality of substrate stages is known, which may be considered to be easy from a theoretical viewpoint. However, there are numerous problems which should be solved in order to exhibit a sufficient effect. For example, if two substrate stages each having a size equivalent to those of presently used substrate stages are merely arranged and placed side by side, an inconvenience arises in that the installation area for the apparatus (so-called foot print) is remarkably increased, resulting in increase in cost of the clean room in which the exposure apparatus is placed. In order to realize highly accurate overlay, it is necessary to execute alignment for the sensitive substrate on an identical substrate stage, and then execute positional adjustment for the image of the pattern on the mask and the sensitive substrate by using a result of the alignment so that exposure is carried out. Therefore, for example, if one of the two substrate stages is merely exclusively used for exposure, and the other is merely exclusively used for alignment, there is no real countermeasure.
Further, there have been hitherto the following necessities. That is, when two operations are concurrently processed in parallel to one another while independently controlling movement of two substrate stages, then the movement should be controlled so that the both stages do not make contact with each other (prevention of interference), and the operation performed on one of the stages does not affect the operation performed on the other stage (prevention of disturbance).
Furthermore, in the case of the scanning type projection exposure apparatus, the order of exposure for respective shot areas on the wafer W is determined, for example, by respective parameters of (1) to (4), i.e., (1) acceleration and deceleration times during scanning, (2) adjustment time, (3) exposure time, and (4) stepping time to adjacent shot. However, in general, the acceleration and the deceleration of the reticle stage give the rate-determining condition. Therefore, it is most efficient that scanning is alternately performed for the reticle stage from one side to the other side and from the other side to one side in the scanning direction, in synchronization with which scanning is alternately performed for the wafer in the direction opposite to that for the reticle stage (for this purpose, the wafer is subjected to stepping in an amount corresponding to one shot after exposure for one shot).
However, when the conventional complete pre-measurement control described above is performed (for example, Japanese Laid-Open. Patent Publication No. 6-283403), it has been difficult to perform exposure in the aforementioned most efficient order of exposure. That is, when a shot area in the vicinity of the center of the wafer is exposed, the complete pre-measurement control can be performed without any special problem. However, in the case of shot areas existing in the vicinity of the outer circumference of the wafer, and in the case of incomplete shots existing on the outer circumference, it is sometimes difficult to perform the complete pre-measurement control depending on the scanning direction for such shot areas. In the present circumstances, it is inevitable to direct the scanning direction from the inside to the outside of the wafer in order to perform complete pre-measurement. For this reason, the throughput has been consequently lowered.
Japanese Laid-Open Patent Publication No. 8-51069, corresponding to U.S. patent application Ser. No. 261,630 filed on Jun. 17, 1994, discloses a step-and-repeat apparatus comprising a plurality of wafer stations each of which comprises a wafer position observing and tracking apparatus. The apparatus is provided, as the wafer station, with an image-forming station and a characteristic measuring station, and each station has a chuck for holding the wafer thereon. On the characteristic measuring station, an inclination and a depth of a field is determined for each field of the wafer. The image-forming station is provided with an image-forming lens, and the image is printed on each field of the wafer on which the characteristic has been measured in the measuring characteristic station. To measure the characteristic and to form an image in these stations are performed in parallel. This publication discloses that therefore the throughput of this apparatus can be doubled compared with a conventional stepper which performs the measurement of characteristic and the formation of image in order. However, in this type of apparatus, in order that the data concerning the wafer collected on the measuring characteristic station are kept effective and accurate even after the wafer has been transferred to the image-forming station, the wafer must be monitored continuously with an interferometer.
The present invention has been made under the circumstances as described above, and a first object of the invention is to provide a projection exposure apparatus which makes it possible to further improve the throughput.
A second object of the invention is to provide a projection exposure method which makes it possible to further improve the throughput.
A third object of the invention is to provide a projection exposure apparatus which makes it possible to improve the throughput by concurrently processing, for example, the exposure operation and the alignment operation in parallel to one another, miniaturize a substrate stage, and reduce the weight of the substrate stage.
A fourth object of the invention is to provide a projection exposure method which makes it possible to improve the throughput, miniaturize a stage, and reduce the weight of the stage.
A fifth object of the invention is to provide a projection exposure apparatus which makes it possible to further improve the throughput and avoid any mutual influence of disturbance between the both stages.
A sixth object of the invention is to provide a projection exposure apparatus which makes it possible to further improve the throughput and avoid any mutual interference between the both stages.
A seventh object of the invention is to provide a projection exposure method which makes it possible to further improve the throughput and avoid any mutual influence of disturbance between the both stages.
An eighth object of the invention is to provide a projection exposure method which makes it possible to further improve the throughput and avoid any mutual interference between the both stages.
A ninth object of the invention is to provide a projection exposure apparatus which makes it possible to perform highly accurate focus/leveling control while further improving the throughput.
A tenth object of the invention is to provide a projection exposure method which makes it possible to perform highly accurate focus/leveling control while further improving the throughput.
An eleventh object of the invention is to provide a projection exposure method which makes it possible to perform highly accurate focus/leveling control while further improving the throughput even when EGA is performed for conducting positional adjustment with respect to a mask on the basis of an arrangement of sample shot areas.
A twelfth object of the invention is to provide a projection exposure apparatus which makes it possible to perform highly accurate focus/leveling control while further improving the throughput, such that focus information concerning those disposed at the inside, which has been impossible to be subjected to pre-measurement when shot areas in the vicinity of outer circumference of a sensitive substrate are exposed, is used as pre-measurement data for focus control.
A thirteenth object of the invention is to provide a scanning exposure method which makes it possible to perform highly accurate focus/leveling control while further improving the throughput.
A fourteenth object of the invention is to provide an exposure method capable of improving throughput and determining the size of the substrate stage regardless of the baseline amount.